The upper mirror is curved to prevent beam walk-off in the cavity, leading to betterstability of the lasing mode.To conduct the heat away from the bottom mirror, a hole is etched in the InPsubstrate. The design uses a 980 nm pump laser to pump the VCSEL cavity. Anypump wavelength lower than the desired lasing wavelength can be used to excite thesemiconductor electrons to the conduction band. For example, the 980 nm semicon-ductor pumps used to pump erbium-doped fiber amplifiers can be used here as well.By designing the pump spot size to match the size of the fundamental lasing mode,

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3.5Transmitters187the laser can be made single mode while suppressing the higher-order Fabry-Perotcavity modes. Using gain to perform this function is better than trying to design thecavity to provide higher loss at the higher-order modes. The high gain also allows theoutput coupling reﬂectivity to be reduced, while still maintaining sufficient inversioninside the cavity to prevent excessive recombination.The laser described in [Vak99] was able to put out about 0 dBm of power incontinuous-wave (CW) mode over a tuning range of 50 nm.Two- and Three-Section DBR LasersWe saw earlier that we can change the refractive index of a semiconductor laser byinjecting current into it. This can result in an overall tuning range of about 10 nm.The DFB laser shown in Figure 3.44 can be tuned by varying the forward-biascurrent, which changes the refractive index, which in turn changes the effective pitchof the grating inside the laser cavity. However, changing the forward-bias currentalso changes the output power of the device, making this technique unsuitable foruse in a DFB laser.A conventional DBR laser also has a single gain region, which is controlled byinjecting a forward-bias currentIg, as shown in Figure 3.44(b). Varying this currentonly changes the output power and does not affect the wavelength. This structurecan be modified by adding another electrode to inject a separate currentIbinto theBragg region that is decoupled from the gain region, as shown in Figure 3.52(a). Thisallows the wavelength to be controlled independently of the output power.As in a conventional DBR laser, the laser has multiple closely spaced cavity modescorresponding to the cavity length, of which the one that lases corresponds to thewavelength peak of the Bragg grating. As the wavelength peak of the grating is variedby varyingIb, the laser hops from one cavity mode to another. This effect is shownin Figure 3.52(a). As the currentIbis varied, the Bragg wavelength changes. At thesame time, there is also a small change in the cavity mode spacing due to the changein refractive index in the grating portion of the overall cavity. The two changes donot track each other, however. As a result, asIbis varied and the Bragg wavelengthchanges, the laser wavelength changes, with the laser remaining on the same cavitymode for some time. As the current is varied further, the laser hops to the next cavitymode. By careful control over the cavity length, we can make the wavelength spacing

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